I've already managed to hack the experience formula. I will tell you right now, it's HARD. It's not laid out all nice and simple for you to find.
However, keeping in with this thread , I CAN tell you where the game writes the totalled up experience to (That is, the result of the Experience Formula).
I can't remember where in the rom exactly this happens, but the game writes the experience gain to 0x02023D50. I hope this will help people to come up with new and interesting ways of using exp gain. This is for the battle exp gain formula. The experience gaining types (slow, erratic etc) all use a LOOK UP table. I don't have the location of said table unfortunately, so you'll have to look it up yourself.
Hope this helps! :D